This invention relates to an olefinic block copolymer which has good strength and resistance to heat and low-temperature impact as well as excellent appearance and processability.
In recent years, flexible resin materials having excellent strength and resistance to heat and impact (especially at low temperature) have been demanded for the parts or materials to be used in automobiles, exteriors, household electric appliances, electric cables, pipes, sheets, etc. Conventional soft polymeric materials include plasticized polyvinyl chloride, ethylene-vinyl acetate copolymer, vulcanized rubber, etc. They generally have excellent impact resistance at low temperature but have poor resistance to heat or to cold temperature. Thus, their uses have been restricted. A random copolymer of propylene and other olefins normally has good thermal resistance, but is somewhat deficient in flexibility, and has poor impact resistance at low temperature, in comparison with the above mentioned polymeric materials. The resin material which somewhat satisfies the above mentioned properties simultaneously can be obtained, for example, by mechanically blending polypropylene and ethylene-propylene rubber or the like. Such blended resin materials, however, are not attractive because of cost increase in such blending steps.
The term "flexible resin materials" according to the present invention means the materials having Olsen flexural rigidity (10-degree angle) of not more than 1,500 kg/sq.cm and strictly not more than 1,000 kg/sq.cm, or JIS-A hardness of not more than 97 and strictly not more than 95.
Instead of mechanically blending polypropylene and ethylene-propylene rubber, it has been proposed to polymerize them in a polymerization reactor according to two-step polymerization method. The resulting block copolymer is advantageous in costs in comparison with the above mentioned mechanically blended product, and also has the physical properties of such blended materials.
As far as we know, block copolymers comprising propylene and ethylene and satisfying the flexibility sought in the present invention are disclosed in Japanese Patent Laid-Open Publication Nos. 80418/80, 10611/82 and 10612/82. These block copolymers are composed of two blocks. The results of our experiments, however, indicate that molded articles of these block copolymers have low commercial values because flow marks (i.e. streaks periodically appearing perpendicularly to the direction of resin flow) are formed on the surfaces thereof when the copolymers are injection-molded, and these articles are also markedly whitened at deformed portions when subjected to impact or deformation. Extruded articles of these block copolymers also have poor properties because the tensile strength thereof at fracture is markedly low at high temperatures, although tensile characteristics at room temperature are fairly good.